Piezoelectric actuators are used with increasing frequency in a variety of applications, notably in fuel injectors. One common design utilizes a piezoelectric actuator to control the position of a control valve for controlling injection of fuel via a fuel injector. Piezoelectric actuators have been demonstrated to have certain advantages over conventional actuator systems such as solenoid actuators. In particular, piezoelectric actuators tend to be capable of relatively precise and repeatable operation under the demanding conditions commonly associated with fuel injector operation. The often superior performance of piezoelectric actuators has resulted in their displacing conventional actuators in certain types of fuel systems.
Despite the advantages offered by piezoelectric actuators, a unique set of challenges has arisen in connection with implementing piezoelectric actuators in commercially viable fuel injection systems. For example, it may be necessary to apply a preloading force to piezoelectric elements used in piezoelectric actuators so that they can function properly. In other words, piezoelectric elements are typically held in compression within an actuator subassembly or the like, and elongate against the compressive force when an electrical potential is applied to the piezoelectric element. Engineers have struggled with developing effective means for applying and maintaining a proper preload on piezoelectric elements. Piezoelectric actuators also tend to experience thermally induced dimensional changes when in service.
Many materials used in constructing piezoelectric actuators will tend to expand as their temperature increases. A preloading force applied to a piezoelectric element via a spring, etc., can therefore change as the piezoelectric actuator changes temperature. In many instances, it is desirable to set a preload on a piezoelectric element relatively precisely to ensure consistent operation. Because inconsistency in actuator operation can affect the ability of a fuel injector to function properly, potentially disrupting sophisticated injection timing strategies, variations in preload due to temperature changes can compromise overall engine performance.
U.S. Pat. No. 6,983,895 to Augustine et al. (“Augustine”) is directed to a piezoelectric actuator having a compensator. In particular, a compensator is used in the piezoelectric actuator of Augustine which expands to compensate for a parallel expansion of a housing for the actuator. Augustine recognizes the importance of minimizing the effects of thermal expansion on the actuator, and may be beneficially implemented in certain environments. In other instances, however, particularly at certain temperature ranges or when using certain materials in construction of the actuator, Augustine may be ineffective.